1. Field of the Invention
The invention relates to a vehicular steering control apparatus. More specifically, the invention relates to a vehicular steering control apparatus that controls an automatic steering unit for automatically steering steerable wheels relative to a steering input unit and a steering assist force generation unit for generating a steering assist force in accordance with an operational state of a vehicle that has the automatic steering unit and the steering assist force generation unit.
2. Description of the Related Art
As one vehicular steering control apparatus for an automobile or the like, as disclosed for example in Japanese Patent Application Laid-Open No. 5-77751, there is known a steering control apparatus that performs active steering control for automatically steering steerable wheels and steering assist force control for counterbalancing a steering counterforce resulting from the automatic steering of the steerable wheels through active steering control. Japanese Patent Application Laid-Open No. 2000-229579 discloses control for reducing a steering counterforce in an active steering system.
According to the steering control apparatuses of the related art as mentioned above, since a steering counterforce resulting from the automatic steering of the steerable wheels through active steering control is counterbalanced, fluctuations in the steering counterforce resulting from active steering control can be damped, so that the feeling of steering can be improved. In the case where automatic steering based on active steering control is performed, however, if the actual turning direction of the steerable wheels that are turned through automatic steering is inverted, the direction of application of a frictional force in a steering system located on the output side with respect to an active steering mechanism is inverted, whereby the steering counterforce changes abruptly. In this respect, therefore, there is a problem of unpleasant feelings during steering.
Further, it has also been known to calculate a feedforward control amount consisting of an inertia term, a damper term, a spring term, and a friction term of a steering system as a control amount for counterbalancing a steering counterforce, and to perform steering assist force control by controlling a power steering unit on the basis of the sum of the feedforward control amount and a feedback control amount based on a steering torque. However, since the magnitude of a frictional force in a steering system and the timing of generation thereof cannot be estimated precisely, even the feedforward control amount including the friction term cannot prevent the steering counterforce from changing abruptly due to the inversion of the direction of application of the frictional force in the steering system.